The present invention relates to the use of Barkhausen noise to determine stress within the interior of a ferromagnetic specimen by creating both a cyclic and a spatially-varying bias magnetic fields within the specimen. By time-gating a detected Barkhausen noise signal, a tomographic map of stress within the ferromagnetic specimen can be produced.
The interior stress in many components are often of interest in determining whether the component can perform its function without failure. Non-destructive evaluation (NDE) methods for stress measurement include x-ray diffraction, ultrasonic bi-refringence, magnetically introduced ultrasonic velocity change, and Barkhausen noise. Barkhausen noise can be sensed either inductively or acoustically. The approaches used in the past, however, are limited either to measurement of stress at or near the surface, or measurement of the average bulk stress throughout a volume of material. The present invention shows a new approach which allows the internal stress to be determined at specific regions in the interior of the ferromagnetic material using acoustically detected Barkhausen noise.
The Barkhausen effect is caused by the discontinuance and irreversible motion of magnetic domain walls as a ferromagnetic material is magnetized. It is well known that the Barkhausen activity is influenced by the state of the mechanical stress in the material, and can be used to measure the residual or applied stress. The Barkhausen noise may be sensed as either a burst of voltage pulses introduced in the pick-up coil or as ultrasonic pulses detected by an acoustical transducer. The inductive detection method is limited to stress measurements in only the near surface region since the skin effect causes attenuation of the magnetic disturbances associated with the domain wall motion deeper in the interior of the material. The acoustical pulses originating in the interior of the material are not strongly attenuated with distance, and, therefore, the acoustical detection method offers the possibility of interior stress measurement.
The use of the Barkhausen effect to determine stress is explained in more detail in "Barkhausen Effect--An Indication of Stress," Materials Evaluation, Vol. 28, No. 7, July 1970, pages 157-61, which article was written by an employee of Southwest Research Institute, the assignee of the present invention. Also, U.S. Pat. No. 3,783,370 by Birdwell shows a feedback system for amplifying the Barkhausen signal when doing an NDE test. Neither of these references show the use of cyclic and spatially-varying bias magnetic fields that produce a time-gated signal for determining the Barkhausen noise in a specific interior region of a ferromagnetic specimen. It is the measurement in the interior region that allows for the tomographic mapping of the stress throughout the specimen.
A basic discussion of Barkhausen theory and the basic method and apparatus for NDE investigation of a ferromagnetic specimen by means of measuring Barkhausen noise can be found in U.S. Pat. No. 3,427,872 entitled "Method and System for Investigating the Stress Condition of Magnetic Materials" issued Feb. 18, 1969, to R. W. Leep and Richard L. Pasley, which is hereby incorporated by reference.
The Barkhausen noise is typically generated by applying a cyclic magnetic field to the material; however, this causes acoustic Barkhausen noise to be produced at the same time throughout the entire magnetized region of the material. Therefore, it is not possible to distinguish the noise from specific regions. The present invention describes a new approach which uses a combination of cyclic and spatially-varying magnetic fields to produce Barkhausen noise from specific regions in the specimen at specific times in the magnetization cycle. By time-gating the acoustic Barkhausen signal, stress can be measured in specific regions in the interior of the specimen by measuring the gated signal amplitude. The approach offers the possibility of a tomographic measurement of stress distribution throughout the interior of a specimen.